The present invention relates to a mechanical sensor and, in particular, to a mechanical sensor for operating a spring-operated pretensioner of a vehicle seat belt system.
In a seat belt system provided on a vehicle, such as an automobile, a pretensioner for tensioning the belt is furnished in some cases for more securely restraining the body of an occupant in an emergency. There are various types of pretensioners, such as those that rotate the reel of the belt retractor in the belt-winding direction, those that act directly on the belt to form a loop and those that pull the buckle in a direction to tighten the belt. As the driving mechanism for such pretensioners, it is known to use a piston-cylinder operated by gunpowder or to use a spring.
A driving mechanism using gunpowder requires an electric trigger to operate, which is costly to provide and install. For this reason, a spring-operated pretensioner with a purely mechanical structure is preferred.
However, the driving mechanism to operate the pretensioner must have a sufficient operating stroke and produce the required operating force. When a spring is used in the driving mechanism, a powerful spring must be used. The preload for the spring is high, and a considerable operating force is required to release the preload. Accordingly, the output force of a trigger for operating the pretensioner also must be high.
Mechanical sensors are disclosed in Provisional Japanese Utility Model Publications No. 90159/1982 and No. 33060/1983, Japanese Utility Model Publication No. 35017/1988, and the Provisional Japanese Patent Publications No. 206765/1983, No. 164650/1989, and No. 164651/1989. According to all of these disclosures, an inertial body directly supported by a trigger is moved by inertial force to release the device.
In order to obtain a sufficiently high trigger-operating force by a conventional type mechanical sensor as described above, the inertia body for detecting acceleration must have higher inertial force, which requires a large and heavy sensor. Also, the sensor is incorporated in the pretensioner and when it is mounted on the seat of a vehicle, it is subject to the vibration of seat or seat sliding. This places a restriction on the location of the sensor.